The reactions between olefin compounds, carbon monoxide and hydrogen in the presence of a catalyst to give the aldehydes comprising one additional carbon atom are known as hydroformylation or oxo synthesis. The catalysts used in these reactions are frequently compounds of the transition metals of group VIII of the Periodic Table of the Elements. Known ligands are, for example, compounds from the classes of the phosphines, phosphites and phosphonites, each with trivalent phosphorus PIII. A good overview of the state of the hydroformylation of olefins can be found in B. CORNILS, W. A. HERRMANN, “Applied Homogeneous Catalysis with Organometallic Compounds”, vol. 1 & 2, VCH, Weinheim, N.Y., 1996 or R. Franke, D. Selent, A. Bönier, “Applied Hydroformylation”, Chem. Rev., 2012, DOI:10.1021/cr3001803.
Every catalytically active composition has its specific benefits. According to the feedstock and target product, therefore, different catalytically active compositions are used.
The disadvantage of bi- and polydentate phosphine ligands is a relatively high level of complexity necessary for preparation thereof. It is therefore often unfeasible to use such systems in industrial operations. An additional factor is comparatively low activity, which has to be compensated for by chemical engineering, through high residence times. This in turn leads to unwanted side reactions of the products.
In Angew. Chem. Int. Ed. 2000, 39, No. 9, p. 1639-1641, Börner et al. describe ligands having one P—C bond and two P—O bonds; these are thus phosphonites. The phosphonites described therein, when used in hydroformylation, have n/iso selectivities (n/iso=the ratio of linear aldehyde (=n) to branched (=iso) aldehyde)) of 0.61 to 1.57.
The phosphonite ligands described in DE 199 54 721 have a good n/iso selectivity. However, in-house studies have shown that the compound II-c (in DE 199 54 721; page 6) has a tendency to photochemically induced breakdown, and should therefore not be used on the industrial scale.
One disadvantage of the ligands having a phosphonite structure is that their preparation is very complex. However, the possibility of a favourable and simple synthesis is crucial for the use of ligands in an industrial scale process.
Ease of availability and the associated good possibility of industrial scale use is an important criterion, since the preparation complexity and the associated production costs for the ligand may only be so high that the viability of the overall process in which the ligand is to be used at a later stage is still assured.
Rhodium-monophosphite complexes in catalytically active compositions are suitable for the hydroformylation of branched olefins having internal double bonds.
Since the 1970s, there have been descriptions of the use of “bulky phosphites” in hydroformylation (see, inter alia, van Leeuwen et al., Journal of Catalysis, 2013, 298, 198-205). These feature good activity, but the n/i selectivity for terminally hydroformylated compounds is in need of improvement.
EP 0 155 508 discloses the use of bisarylene-substituted monophosphites in the rhodium-catalysed hydroformylation of sterically hindered olefins, e.g. isobutene. However, rhodium concentrations used here are sometimes very high (one being 250 ppm), which is unacceptable for an industrial scale process in view of the current cost of rhodium and has to be improved.
For hydroformylation reactions, tris(2,4-di-tert-butylphenyl)phosphite (TDTBPP) is currently one of the best-performing monophosphite ligands commercially available, and is available under the Alkanox 240 trade name (see also R. Franke, D. Selent, A. Börner, “Applied Hydroformylation”, Chem. Rev., 2012, 112, p. 5681, chapter 3.4.2).